Queries with a known search condition

When the exact start and stop conditions are known at compilation time,
the optimizer uses the index itself to make a very precise estimate of the
number of rows that will be scanned from disk. An example of a query with
a known search condition:

SELECT *
FROM Flights
WHERE orig_airport = 'SFO'

The search value, 'SFO', is known. The optimizer will be able to make an
accurate estimate of the cost of using the index orig_index.

In addition, if the index is unique, and the WHERE clause involves an =
or IS NULL comparison to all the columns in the index, the optimizer knows
that only a single row will be scanned from disk. For example: